This invention relates generally to a quantitative thickness measurement for an electrically insulative metal oxide coating that has been deposited on a metal substrate. The present method is particularly useful in determining the thickness of extremely thin oxide coatings deposited on metal substrates having a difficult shape to measure by direct means such as wires and the like. For example, the oxidized coating on dumet wires that are used as the lead-in conductors in various type electric lamps has been conventionally determined for thickness by visual inspection of the color of the oxide coatings. Such an indirect thickness evaluation is understandably not very precise and leads to undesired variation when the lamps are subsequently manufactured. It would be desirable, therefore, to measure exact thickness of such oxide coatings and in a manner which further lends itself to automated measurement.
The electrolytic reduction of an oxide coating in an electrochemical cell is known. Thickness of the oxide coating can be calculated from the known Faraday relationship between quantity of electricity used, the sample surface area, electrochemical equivalent of the oxide used, and the density of the particular oxide composition. In general, oxide coatings in the range of 10-100 microinches can be measured in this manner accurately within 20-140 seconds, respectively, with existing test equipment. For example, thickness measurement of a cuprous oxide coating on a dumet wire can be determined in an electrochemical cell having a 0.1 normal potassium chloride electrolyte as the reducing agent using the above indicated Faraday equation. For conventional measurement of the oxide coating thickness in this manner, however, the services of a skilled chemist or technician are generally required.
It would be desirable to provide a method and apparatus suitable for automatically measuring metal oxide coating thickness on an electrically conductive metal substrate in a precise and convenient manner. It would be further desirable to do so in a manner which controls test measurement as a function of the sample dimensions and which automatically terminates the test procedure when the oxide coating thickness has been determined.